1. Field of the Invention
The present invention relates to improved lentiviral vectors and their use in gene delivery and high level expression of desired transgenes to target cells, particularly to differentiated blood lineages derived from lentiviral vector-modified human hematopoietic stem cells (hHSC).
2. Description of Related Art
Gene therapy via the transduction of human hematopoietic stem cells (hHSC) represents a very promising approach for the treatment of a number of inherited and acquired lympho-hematological disorders. The stable genetic manipulation of long term repopulating hHSC with existing gene delivery systems, however, has been impossible to achieve at an efficiency compatible with therapeutic realities. Oncoretroviral vectors derived from Moloney murine leukemia virus (MLV), for instance, although highly appealing since they integrate their cargo into the chromosomes of target cells, cannot transduce HHSC that have not been first treated with inducers of proliferation (Kohn et al., 1991; Mazurier et al., 1998). Indeed, the nuclear transport of the MLV preintegration complex requires the breakdown of the nuclear envelope that occurs at mitosis (Roe et al., 1993; Lewis and Emerman, 1994). Unfortunately hHSCs, whether harvested from the bone marrow (BM), the umbilical cord blood (UCB) or mobilized in the peripheral circulation, are mostly non-dividing and lose their pluripotentiality after continuous stimulation and proliferation (Bhatia et al., 1997; Dao et al., 1997; Dorrell et al., 2000). Recent reports, however, have shown that a significant fraction of pluripotent cells as well as cells capable of long-term engraftment in non-obese diabetic/severe combined immunodeficient (NOD/SCID), also called SCID-repopulating cells (SRC), can be maintained, transduced and even expanded using specific stimulation conditions (Dorrell et al., 2000; Dao et al., 1998; Piacibello et al., 1999; Ueda et al., 2000).
Lentiviruses are a subgroup of retroviruses that can infect non-dividing cells owing to the karyophilic properties of their preintegration complex, which allow for its active import through the nucleopore. Correspondingly, lentiviral vectors derived from human immunodeficiency virus type 1 (HIV-1) can mediate the efficient delivery, integration and long-term expression of transgenes into non-mitotic cells both in vitro and in vivo (Naldini et al., 1996a; Naldini et al., 1996b; Blomer et al., 1997). In particular, HIV-based vectors can efficiently transduce human CD34+ hematopoietic cells in the absence of cytokine stimulation (Akkina et al., 1996; Sutton et al., 1998; Uchida et al., 1998; Miyoshi et al., 1999; Case et al., 1999). These cells are capable of long-term engraftment in NOD/SCID mice (Miyoshi et al., 1999). Bone marrow from these primary recipients can repopulate secondary mice with transduced cells, confirming the lentivector-mediated genetic modification of very primitive hematopoietic precursors, most probably bona fide stem cells. Since none of the other currently available gene delivery systems has such an ability, lentiviral vectors provide a previously unexplored basis for the study of hematopoiesis and for the gene therapy of inherited and acquired lympho-hematopoietic disorders via the genetic modification of HSCs.
The demonstration of this important point, however, was provided with an early generation of lentiviral vectors unsuitable for therapeutic applications, either because they failed to meet biosafety requirements (Akkina et al., 1996; Sutton et al., 1998; Uchida et al., 1998) or because they induced levels of transgene expression that were dismissingly low (Miyoshi et al., 1999; Case et al., 1999; An et al., 2000). Accordingly, there is a significant need to develop improved lentiviruses for use as transducing vectors that are capable of effectively transducing hematopoietic cells, particularly hematopoietic progenitor cells, and which are capable of expressing desired transgenes at high levels.
An optimal stem cell gene therapy approach should result in the efficient transduction of HSCs, and considering the plasticity of stem cells, in the restricted expression of therapeutic genes into specific mature blood cell lineages. Third generation lentiviral vectors are currently the most optimized tools for gene delivery into non-cycling human HSC. Moreover, a self-inactivating design (SIN) provides for the use of tissue specific promoters without interference from the upstream LTR.